THE CATERPILLAR AND THE MOTH 



fore, are differentiated in the embryo into two sets of cells, 

 one set of which will form the special organs of the larva, 

 while those of the other will remain dormant during the 

 larval life to form the adult organs when the larval cells 

 have completed their functions. The cells of the second 

 set carry the hereditary influences that will cause them to 

 develop into the original, or ancestral, form of the species; 

 the cells of the first set produce the temporary larval 

 form, which may retain certain primitive characters from 

 the embryonic stage, but which does not represent an 

 ancestral form in the evolution of the species. 



An extreme case of anything is always more easily 

 understood when we can trace it back to something simple, 

 or link it up with something familiar. The metamorphosis 

 of insects appears to be one of the great mysteries of 

 nature, but reduced to its simplest terms it becomes only 

 an exaggerated case of a temporary growth in certain 

 groups of cells to form something of use to the young, 

 which disappears by resorption when the occasion for its 

 use is past. Innumerable simple cases of this kind might 

 be cited from insects; but there is a familiar case of well- 

 developed metamorphosis even in our own growth, 

 namely, the temporary development of the milk teeth and 

 their later substitution by the adult teeth. If a similar 

 process of double growth from the somatic cells had been 

 carried to other organs, we ourselves should have a meta- 

 morphosis entirely comparable with that of insects. 



The Moth 



For three weeks or a little longer the processes of re- 

 construction go on within the pupa of the tent caterpillar, 

 and then the creature that was a caterpillar breaks through 

 its coverings and appears in the form and costume of a 

 moth (Fig. 1^9 J). The pupal shell splits open on the 

 forward part of the back (E) to allow the moth to emerge, 

 but the latter then only finds itself face to face with the 

 wall of the cocoon. It has left behind its cutting instru- 



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